Active biasing improves stereo performance

The use of digital potentiometers and
volume-control ICs to replace mechanical pots enables a wider
variety of user-interface features, but consumer-product designers
are also pushed to reduce costs at every turn. To achieve the best
audio performance you must pay careful attention to circuit
details, which sometimes requires a tradeoff between performance
and cost. The following discussion compares active and passive
biasing circuits for digital potentiometers, considers their
performance, and provides equations for making the right tradeoffs
in your design.

Volume controls and digital potentiometers

Figure 1
illustrates the circuitry in question. We focus on single-supply
applications, which are common in products powered by batteries and
wall adapters. In a single-supply application, all parts of the
system are powered between VDD
and ground, and the signals swing between VDD and ground. You can use capacitors
between the stages, or eliminate them altogether for reasons of
cost and performance.

Fig. 1. The wiper buffer reduces
distortion by reducing current through the switch array internal to
the digital potentiometer.

A digital-potentiometer IC (outlined in red) is
an array of resistors and switches under the control of logic that
emulates the sliding contact wiper of a mechanical potentiometer. A
volume-control IC (outlined in blue) is a digital-potentiometer IC
that incorporates two other circuits very important for best audio
performance — an op-amp buffer for the wiper, and a bias
generator for producing VBIAS.

Passive bias circuit

To control costs while using a digital
potentiometer, you can generate the bias voltage with a passive
resistive divider (see Fig. 2a).
Equal resistor values set VBIAS
at the midpoint between VDD and
ground. To reduce the ac impedance and noise at VBIAS, you should also add a bypass
capacitor (C2, Fig. 2b).

Fig. 2. Passive VBIAS: basic (a), with noise suppression
(b).

Let’s consider how the chosen circuit
values affect audio performance. Source impedance for the bias
circuit, sometimes called the stiffness of the supply, is
calculated using Equation (1):

Z2 =(R2 x 1/sC2)/(R2 + 1/sC2) =

R2/(R2 x sC2 + 1) Equation (1)

Zin = (Z1 x Z2)/(Z1 + Z2) =

[R1 x R2/(R2
x sC2 + 1)]/[R1 + R2/(R2
x sC2 + 1) =

R1 x R2/(R1
x R2 x sC2 + R1 + R2) Equation (2)

At dc (s = 0), note that equation 2 reduces to
the impedance of R1 and R2 in parallel. We now insert this
impedance into the volume-control circuit (see Fig. 3).

Fig. 3. Circuit model for the
analysis.

Note that with the wiper at the low (L) end of
the digital potentiometer, the finite source impedance in the bias
network allows a signal to appear on the L terminal of the digital
potentiometer. Yet, setting the wiper to L is supposed to mute the
audio channel! Instead of no signal, as desired, we see a
divided-down version of the source voltage, VIN. For example, with two
10-kΩ bias resistors and a 40-kΩ pot with wiper
at the L position, we see an output voltage of

VOUT = 5
kΩ/(40 kΩ + 5 kΩ) x VIN

which is only −19 dB below full scale
(dBFS) at dc.

What happens if we include C2 in the analysis? Substituting 0.01
µF for C2 in equation 2, the result is shown in Fig. 4. We had almost no effect below 1
kHz, and the effect at 20 kHz is to reduce the impedance to just
785 Ω. We achieve a mute attenuation of just –22
dB, as shown in Fig. 5. The
capacitor can be raised to 10 µF (large). At 100 Hz we
now have a mute attenuation of just −36 dB. This is not
even close to the ultimate performance of the volume control, nor
is it close to a reasonable specification for mute.

Although we analyzed this problem with the wiper
sitting at the mute position (the L terminal), it’s
apparent that a finite source impedance for VBIAS affects all potentiometer
settings. As you approach the low end of the potentiometer, that
effect increases inaccuracy in the attenuation curve.

We need to reduce the impedance, and increase
the muting to a level of 90 dB or so. To reach 90 dB, the source
impedance for VBIAS must be in
the range of single-digit ohms. You can reduce this impedance by
using smaller values for R1 and
R2, at the expense of higher dc
current, but that approach isn’t practical. Obviously,
with this passive circuit, we must depend on C2 to get the impedance very low before
entering the audio band. To achieve 95 dB at 100 Hz, for example,
you find that the capacitor value is again unrealistic. For values
of 10 kΩ, 10 kΩ, and (a somewhat large) 100
µF, the attenuation is still only moderately effective.
The solution to this problem is an active circuit.

Before discussing the active circuit, consider a
stereo design. For left and right signals sharing a passive bias
generator, we not only have the problems of muting and feedthrough,
but the additional problem of crosstalk as well. Crosstalk is the
leakage of signal from the left (L) channel into the right (R)
channel, and vice versa. The circuit for a shared bias line appears
as in Fig. 6.

Fig. 6. Stereo volume control, sharing
VBIAS.

Finite bias impedance creates a signal voltage
on the L terminal in response to an input on the H terminal, and
the shared bias ensures signals at both VOUT pins from an input on either the
left or the right. This in-channel signal appears as poor muting
attenuation (a failure to follow the attenuation curve). The
opposite-channel signal appears as crosstalk or a loss of stereo
separation.

Active bias

The solution to all of these problems is to
provide a very stiff (low-impedance) source for VBIAS, as shown in Fig. 7. The divided-down VDD is buffered by an op amp whose
closed-loop output impedance at dc is a fraction of an ohm. With
careful design, you can then achieve muting attenuation in the
90-dB range.

A test board allows measurements that compare
the passive and active approaches. Figure
8 illustrates the typical performance achieved for both
passive and active circuits, using a test board that includes a
dual audio-taper digital potentiometer for stereo applications
(MAX5457). The passive circuit includes two 1-kΩ resistors
bypassed by 4.7 µF, which produces a pole at 68 Hz
(calculated), and a continuous battery drain of 2.5 mA.

The active circuit can have higher-value
resistors, because its only load is the non-inverting buffer. Thus,
two 100-kΩ resistors hardly affect the active-bias curve,
and their continuous battery drain is just 25 µA.

Integrate it

As we have seen, passive circuits add cost and
size while yielding poor performance. An active circuit that
includes a buffered version of the resistor divider works well, but
adds the overhead of an op amp. A better alternative is a
volume-control IC like the MAX5486 shown in Fig. 9, which integrates an op amp with
the digital potentiometer.

By choosing various members of this IC family,
you can implement a direct interface to a microprocessor,
pushbuttons, rotary encoders, or even infrared remote controls.
Synchronized zero-crossing wiper movements and other features
optimize these devices for audio applications.